Stronger tree size-mortality association with increasing droughts and basal areas: A meta-analysis

Given the escalating tree death rates with increasing drought disturbances, it’s imperative to enhance the mortality predictions for more informed management strategies to preserve forest resources. Tree size could serve as an indicator of tree mortality under drought, as theoretical studies have demonstrated that larger trees, due to their more susceptible hydraulic structures, are more likely to die during droughts. However, empirical studies have presented inconsistent findings regarding the relationship between tree size and mortality, challenging the importance of tree size in projecting drought-induced tree mortality. The variability in this relationship has not been thoroughly quantified, particularly under increasing drought impacts, such as greater drought magnitudes (including intensity and duration), warming temperature and larger stand basal area. We compiled 52 case studies at a global scale and conducted a meta-analysis to explore how the correlation between tree size and mortality fluctuates under drought. Our results showed that: (1) The correlation coefficients of tree size-mortality varied significantly across the studies analyzed, and ∼ 40 % of them reported a positive correlation, i.e., larger trees experienced higher mortality; (2) The correlation coefficients increased with drought intensity, drought duration, maximum temperature during drought, and stand basal area. Our results suggest a stronger association of tree size with tree mortality under intensified and prolonged droughts and greater basal areas. Thus, with the increasing droughts under climate change, tree size tends to play an increasing important role in projecting tree mortality. Moreover, management practices, e.g., thinning, could manipulate basal area to influence the tree size-mortality relationship.